1. Technical Field
This invention relates generally to battery charging and protection circuits, and more specifically to battery charging circuits incorporating a trickle charge during battery undervoltage conditions.
2. Background Art
Battery protection circuits are well known in the art. Protection circuits for rechargeable cells are prolific in battery packs employing lithium-ion and lithium polymer cells. When rechargeable cells are charged, the voltage of the cell rises. Lithium-based rechargeable cells typically have a maximum termination voltage of 4.1 or 4.2 volts. This means that if a lithium-based battery is charged beyond the termination voltagexe2x80x94known as an xe2x80x9covervoltage conditionxe2x80x9d the reliability of the cell may be compromised. Most lithium battery protection circuits known in the art sense the voltage of the cell and terminate charging by opening a switch when the cell reaches the proper termination voltage.
There is another condition, known as an xe2x80x9cundervoltage conditionxe2x80x9d, when the cell voltage drops below it""s recommended operating point. For a typical lithium-ion cell, this voltage is about 2.5 volts. When the voltage drops below this level, possibly due to over discharge, cell manufacturers suggest that rapid charging may damage the cell. Consequently, the battery must be slowly charged until it reaches the minimum operational threshold. The slow charging current, often called a xe2x80x9ctrickle currentxe2x80x9d is on the order of a few hundred milliamps. Once the minimum threshold is reached, a full charging current, like 1 amp for example, may be applied until the maximum termination voltage is reached.
There is thus a need for a compact, low-cost charging circuit that accommodates trickle charging for undervoltage conditions, as well as terminating charging prior to an overvoltage condition occurring.